Only run libc-test on nightly

Otherwise just make sure the library builds on other Rust versions

Author: alexcrichton

.travis.yml

@@ -7,7 +7,11 @@ rust:
 services:
   - docker
 script:
-  - sh ci/run-travis.sh
+  - if [[ $TRAVIS_RUST_VERSION = nightly* ]]; then
+      sh ci/run-travis.sh;
+    else
+      cargo build;
+    fi
 os:
   - linux
   - osx